1. Field of the Invention
The present invention relates to a liquid crystal display panel and a fabrication method of the same, and more particularly to an active matrix type liquid crystal display panel having a sealing member mixed with spacers.
2. Description of the Prior Art
In the active matrix type liquid crystal display panel, liquid crystal is sandwiched between an array substrate and an opposing substrate. The array substrate is provided with pixel electrodes connected to switching element, respectively. The opposing substrate is provided with an opposing electrode. The liquid crystal is sealed within a display area of the panel by using a sealing member provided at peripheral area of the panel. display area
On the array substrate, a plurality of scan lines and a plurality of signal lines, which are crossing mutually, a plurality of pixel electrodes each provided in a different one of cross points of the scan lines and the signal lines, for applying voltages to the respective cross points and a plurality of switching elements such as TFT""s (thin film transistors) for selectively driving the pixel electrodes are provided in matrices through patterning processes. Therefore, there may be some irregularity of an upper surface of the array substrate by the patterning processes.
On the other hand, liquid crystal filling the gap between the array substrate and the opposing substrate constitutes pixel capacitors each having the liquid crystal as a dielectric layer. The liquid crystal display is performed by controlling transmittivity of light correspondingly to electric field intensity of the pixel capacitor, to which a voltage selected by the switching element is applied, by utilizing electro-optical anisotropic property of the liquid crystal.
Therefore, the pixel capacity is varied with variation of thickness (cell gap) of the liquid crystal layer, so that intensity of display is varied. In order to obtain a uniform display quality throughout the display area, it is necessary to improve uniformity of the cell gap throughout the display area. Under the circumstance, various endeavors to uniform the cell gap have been continuing in the technical field to which the present invention belongs.
It has been usual, in order to uniform the cell gap, to use gap maintaining members (spacers) for maintaining the gap between the array substrate and the opposing substrate. As a method for providing the spacers between the array substrate and the opposing substrate, it is usual to arrange them in the liquid crystal in the display area and/or mix them in the seal provided around the display area.
In the case where the spacers are provided in the display area, the spacers may adversely affect the display characteristics. Therefore, this method is not preferable when high display quality is required. That is, in order to constitute a liquid crystal display panel having high display quality, it is effective to maintain the gap between the substrates by removing the spacers arranged in the display area or reducing the number of the spacers.
Furthermore, the variation of the cell gap, which is caused by the surface irregularity of the array substrate resulting from the patterning process, is improved by coating the patterned surface of the array substrate with a soft leveling layer.
FIG. 10 is a partial cross section of a conventional liquid crystal display panel 101 of the type in which a leveling layer is formed on an array substrate and gap retaining members (spacers) are provided in a seal in a mixed relation thereto.
The liquid crystal display panel 101 includes the array substrate 102 as shown in FIG. 10. The array substrate 102 includes a transparent glass substrate 10 and a gate line 11 and a switching element 12 are formed on the transparent glass substrate 10 by patterning. An oxide layer 13, a metal wiring layer 14, the first interlayer insulating layer 15, a light shielding layer 16 and the second interlayer insulating layer 18 are formed on the wafer in the order to constitute a laminated structure including predetermined lamination patterns on the transparent glass substrate 10. A portion of the metal wiring layer 14 constitutes lead wiring lines 14a. 
As shown in FIG. 10, the patterned surface of the array substrate 102, which is the surface of the second interlayer insulating layer 18 in the shown case, becomes irregular. Forming a leveling layer 19 thereon by using spin coating method flattens the patterned, irregular surface of the array substrate 102. On an upper surface of the leveling layer 19, a pixel electrode 20 of ITO (Indium Tin Oxide) is formed. That is, the pixel electrode 20 is an electrically conductive transparent thin layer discretely provided every pixel. The pixel electrode 20 is connected to the metal wiring layer 14 through a contact hole 21 so that the pixel electrode is connectable to the switching element 12.
On the other hand, the liquid crystal display panel 101 includes an opposing substrate 3 as shown in FIG. 10. The opposing substrate 3 is constituted with a transparent glass substrate 30 and at least an opposing electrode 31 formed thereon. It is usual that a light shielding layer and, in a case of a color liquid crystal display device, R, G and B color layers are additionally formed on the transparent glass substrate 30.
After the array substrate 102 and the opposing substrate 3 are completed, respectively, the liquid crystal display panel 101 is assembled. First, a sealing agent 22 is printed on an outer peripheral portion of a display area of either the array substrate 102 or the opposing substrate 3 to seal liquid crystal 130 within a display area. The printing of the sealing agent is performed by the drawing printing in which it is drawn with a single stroke of a seal dispenser or the screen printing. Spacers 23 of a glass material are preliminarily mixed in the sealing agent 22. The sealing agent 22 may be formed of ultraviolet ray setting resin or thermosetting resin.
After the sealing agent printing step, the substrates are stuck together. First, one of the substrates is laid on top of the other and the substrates are positioned such that corresponding cells of the substrates become in opposing relations, respectively. Thereafter, the substrates are pressed by pressing plates to exert pressure on the sealing agent 22 and the spacers 23 to thereby regulate the gap between the substrates to a predetermined value. During this pressing state, the seal agent 22 is hardened. In a case where a ultraviolet setting resin is used as the material of the sealing agent 22, the pressing plates are formed of a material which is transparent for ultraviolet ray and the seal agent 22 is hardened by irradiating the sealing agent 22 with ultraviolet ray through the pressing plates. In a case where a thermosetting resin is used as the material of the sealing agent 22, the seal agent 22 is hardened by heating the sealing agent 22 by a heater provided in the pressing plate or by disposing the liquid crystal display panel in an oven and heating air within the oven.
The liquid crystal display panel 101 which includes the array substrate 102 and the opposing substrate 3 piled on the array substrate and adhered to the array substrate 102 by the seal 4, which contains the spacers 23 and is provided on the outer periphery of the display area A of either one of the array substrate 102 and the opposing substrate 3, is constituted as shown in FIG. 10. The seal 4 is formed of the sealing agent 22 and the spacers 23 mixed therein. The gap between the array substrate 102 and the opposing substrate 3 is maintained uniform owing to the spacers 23. It should be noted that a seal area B is also shown in FIG. 10. The seal area B indicates a common area of the substrates and a space of the gap therebetween, which is covered by the seal 4 provided on either one of the substrates.
There are problems in the above-mentioned conventional technique, which will be described below.
An organic soft material is used for the leveling layer 19. For example, acrylic resin (diethylene glycol ethyl methyl ether) or BCB (benzocyclobutene) is used therefor. On the other hand, the spacer 23 usually takes in the form of a rod or spherical particle of a glass material. Further, the insulating layer (the second interlayer insulating layer 18 in the case shown in FIG. 10), which is an underlying layer of the leveling layer, is formed of a silicon compound such as SiN. That is, the spacers 12 and the underlying layer of the leveling layer are softer than the leveling layer. Therefore, when the array substrate is laid on the top of the opposing substrate and pressed together by the pressing plates, the spacers 23 sink into the leveling layer 19 as shown in FIG. 10. If an amount of sink is uniform throughout the liquid crystal display panel 101, there is no problem in view of the uniformity of cell gap.
However, the amount of sink throughout the panel usually becomes not uniform and tends to be different every portion thereof. When the amount of sink is different every portion, the cell gap in the display area of the liquid crystal display panel varies. As a result, color variation occurs on the display surface, causing the display quality to be degraded.
For a liquid crystal display panel to be used in a projection type liquid crystal display (projector), in which compactness and high display quality of are required, there is a case in which any spacer is not arranged in a display area of the panel and the cell gap is retained by only spacers mixed in the seal. In such case, the influence of the above mentioned problem is considerable.
In view of improvement of producibility of the liquid crystal display panel, it is usual in the recent fabrication method of liquid crystal display panels, to constitute a plurality of liquid crystal display panels on a large substrate and separate them by cutting in a certain step of the fabrication method. This method will be referred to as xe2x80x9cmulti-panel methodxe2x80x9d, hereinafter. The smaller the size of liquid crystal display panel provides the more the number of liquid crystal display panels to be fabricated on a common substrate. There is a case where several tens liquid crystal display panels are fabricated on a common substrate. For example, there is a case, in the multi-panel method, where 64 liquid crystal display panels are formed on a 300 mmxc3x97350 mm substrate simultaneously.
In such multi-panel method, the array substrates as well as the opposing substrates are formed simultaneously on a large substrate provided with a plurality of small substrates, which are arranged in matrix, each corresponding to a unit liquid crystal display panel. When a leveling layer is coated on the large substrate, thickness of the leveling layer becomes different depending upon areas of the large substrate, that is, positions of the small substrates on the large substrate. When the large substrate including the small substrates corresponding to the array substrates are laid on top of the large substrate including the opposing substrates and the large substrates are pressed together, the cell gap varies every small substrate, that is, the liquid crystal display panel, resulting in that the rate of defective products in the plurality of liquid crystal display panels produced from the same large substrates becomes large, that is, the yield, is lowered.
Accordingly, an object of the present invention is to provide a liquid crystal display panel having improved uniformity of cell gap and providing high display quality throughout an image display area thereof.
Another object of the present invention is to provide a method for fabricating the same liquid crystal display panel.
According to the present invention, a liquid crystal display panel comprises a seal member sandwiched between an array substrate and an opposing substrate and a liquid crystal is filled therebetween. The seal member contains spacers in a mixed state for maintaining a gap between the array substrate and the opposing substrate constant. The array substrate is provided with a leveling layer formed such that it covers a switching element array. In order to restrict an amount of sink of the spacers into the leveling layer in an area of the seal member, the leveling layer includes either a thin layer portion thinner than the leveling layer on each switching element of the switching element array or an opening portion.
Particularly, the liquid crystal display panel according to the present invention has a basic structure featured by that an array substrate and an opposing substrate are stuck together such that a seal member containing spacers is provided around a display area of the substrates so as to make a gap between the array substrate and the opposing substrate constant. At least switching elements, a wiring layer and a light shielding layer are formed on the display area of the array substrate as a laminated pattern. The leveling layer is coated on the laminated pattern and pixel electrodes formed on the leveling layer are electrically connected to the respective switching elements through contact holes.
In such liquid crystal display panel, the present invention is featured by that, in order to restrict the sink of the spacers into the leveling layer, the leveling layer includes either thin layer portions or openings are formed.
In an example of the present invention, a bottom-up pattern layer for making an underlying layer of the leveling layer higher is laminated on the seal area of the array substrate. In the case of the liquid crystal display panel 101 shown in FIG. 10, the underlying layer of the leveling layer corresponds to the second interlayer insulating layer 18.
With such construction including the bottom-up pattern layer laminated on the seal area of the array substrate, the level of a lower surface of the leveling layer becomes higher by an amount corresponding to the thickness of the bottom-up pattern layer, so that the thickness of the leveling layer on the seal area can be reduced. Therefore, it is possible to reduce or eliminate the amount of sink of the spacers mixed in the seal into the leveling layer. With reduction or elimination of the amount of sink of the spacers into the leveling layer, the uniformity of sink of the spacers is improved. As a result, the uniformity of cell gap is improved, so that it is possible to maintain the display quality high throughout the image display area.
Furthermore, since the influence of the leveling layer in the seal area is small in even the multi-panel method, there is a merit that the variation of cell gap of every liquid crystal display panel is restricted, so that the yield is improved.
It has been usual that lead wiring layers and insulating layers are laminated on the seal area of the array substrate and the height of the underlying layer of the leveling layer in the seal area is increased thereby. The purpose of the lead wiring layers is to provide an electrically connection to external drive circuits and the purpose of the insulating layers is to electrically insulate between wiring layers. In the present invention, however, the purpose of the bottom-up pattern layer is to raise the underlying layer of the leveling layer. That is, the bottom-up pattern layer is not used for other purpose.
According to the second example of the present invention, the liquid crystal display panel is featured by a construction in which the level of the underlying layer of the leveling layer in the seal area is made substantially equal to a surface of the leveling layer.
In the second example, it is possible to make the thickness of the leveling layer in the seal area very small compared with the average thickness of the leveling layer formed in the display area to thereby substantially restrict the amount of sink of the spacers into the leveling layer. As a result, it is possible to further improve the uniformity of the cell gap and to reliably maintain high display quality throughout the image display area. Even in the multi-panel method, the variation of cell gap of every liquid crystal display panel is restricted and the yield is improved, since the influence of the leveling layer in the seal area is small.
According to the third example of the present invention, the liquid crystal display panel is featured by that the underlying layer of the leveling layer is exposed on the surface of the array substrate in the seal area and the spacers are made in contact with the exposed surface and supported thereby.
That is, in the third example of the liquid crystal display panel, no leveling layer is formed on the seal area in coating the leveling layer or the leveling layer on the seal area is removed after the leveling layer is coated.
In the liquid crystal display panel according to the third example in which there is no leveling layer formed on the seal area and the spacers are in direct contact with the underlying layer of the leveling layer, there is no sink of the spacers into the leveling layer and the uniformity of the cell gap is improved. Therefore, it is possible to reliably maintain high display quality throughout the image display area. Even in the multi-panel method, the variation of cell gap of every liquid crystal display panel is restricted and the yield is improved, since there is no influence of the leveling layer in the seal area.
According to the fourth example of the present invention, the liquid crystal display panel having the construction according to the third example is featured by that a bottom-up pattern layer for level-up of the underlying layer is formed on the seal area of the array substrate.
Therefore, according to the fourth example, there is a merit that a plane for supporting the spacers is not lowered by a value corresponding to the thickness of the bottom-up pattern layer without leveling layer on the seal area of the array substrate, in addition to the merit of the construction according to the third example.
The bottom-up pattern layer may be formed in any of layers constituting the underlying layer of the leveling layer. In fabricating the liquid crystal display panel of the present invention, it may be possible to provide specific forming and patterning processes for the bottom-up pattern layer. However, it is preferable, in view of restriction of increase of the number of fabrication steps, that is, increase of the fabrication cost, to form the bottom-up pattern by utilizing one of the existing patterning processes.
According to the fifth example of the present invention, the liquid crystal display panel having the construction according to the first or fourth example is featured by that the bottom-up pattern layer is in the same layer as that of a predetermined patterned layer formed on the display area of the liquid crystal display panel.
According to the fifth example, it is possible to form the bottom-up pattern layer by utilizing the existing forming and patterning process, without increase of the number of fabrication steps and the fabrication cost.
According to the sixth example of the present invention, the liquid crystal display panel having the construction according to the first or fourth example is featured by that the bottom-up pattern layer is in the same layer as that of the light shielding layer of the display area of the panel.
In the seal area of the panel, there is no need of shielding light by means of the light shielding layer formed in the liquid crystal display panel. In the display area, however, it is necessary to shield the switching elements and the wiring layers against light by providing the light shielding layer of aluminum or aluminum alloy in a periphery of each pixel. In the construction of the liquid crystal display panel according to the sixth example of the present invention, the bottom-up pattern layer is formed by utilizing the forming and patterning process for the light shielding layer of the display area.
According to the seventh example of the present invention, the liquid crystal display panel having the structure according to any of the first to third examples is featured by that the leveling layer is formed of a material having hardness lower than those of the spacers and the underlying layer of the leveling layer.
When hardness of the leveling layer is lower than that of the spacers, the spacers tend to sink into the leveling layer, compared with a case where hardness of the leveling layer is higher than that of the spacers, and, therefore, it is effective to form the underlying layer having hardness higher than that of the leveling layer as in the display panel according to any one of the first to third examples of the present invention. The construction of the liquid crystal display panel according to the seventh example of the present invention results from the above consideration and is effective when the spacers are formed of glass material, the underlying layer of the leveling layer is formed of a silicon compound such as SiN and the leveling layer is formed of a resin material as in the conventional case in which hardness of the leveling layer is lower than those of the spacers and the underlying layer.
According to the eighth example of the present invention, the liquid crystal display panel having the construction according to the first or fourth example is featured by that the bottom-up pattern layer is formed of an electrically conductive material, a plurality of parallel lead wiring lines traversing the seal area are provided in a layer in a level of the layer including the bottom-up pattern layer and an interlayer insulating layer therebetween and the bottom-up pattern layer is cut off at a gap between the lead wiring lines.
That is, according to the construction mentioned above, the reliability of electrical insulation is improved even when the bottom-up pattern layer is formed of an electrically conductive material, since the bottom-up pattern layer is cut off at the gap area between the lead wiring lines.
According to the ninth example of the present invention, the liquid crystal display panel having the construction according to the eighth example is featured by that a distance of the cut-off portion of the bottom-up pattern layer is smaller than a diameter of the spacer.
Therefore, the sink of the spacers into the leveling layer in the ninth example is restricted even in the cut-off portion of the bottom-up pattern layer. When there is no leveling layer in the seal area, the spacers are prevented from dropping in the gap area or cut-off area of the bottom-up pattern. Therefore, the effect of the first invention is obtained reliably throughout the seal area. Further, there is a merit that the uniformity of the cell gap can be improved by effectively functioning an increased number of spacers as the gap maintaining members.
According to the tenth example of the present invention, the liquid crystal display panel having the construction according to one of the first, fourth and eighth examples is featured by that a plurality of parallel lead wiring lines traversing the seal area are provided and the bottom-up pattern layer is formed in the same layer as that of the lead wiring lines and on the gap area between the lead wiring lines.
Therefore, according to this construction, it is possible to relax irregularity caused by the lead wiring lines and to further flatten the laminated pattern for supporting the spacers in the seal area. Consequently, it is possible to make the level of the spacers of the respective panels more flat with respect to the substrate to thereby improve the uniformity of the cell gap by effectively functioning more spacers as the gap maintaining members.
According to the eleventh example of the present invention, the liquid crystal display panel having the construction according to the fourth example is featured by that the seal is adhered to the bottom-up pattern layer and the bottom-up pattern layer is formed with slits.
Therefore, according to this construction, there is a merit that adhesiveness of the seal is improved since the seal agent enters into the slit of the bottom-up pattern layer.
According to the twelfth example of the present invention, the liquid crystal display panel having the construction according to the eleventh example is featured by that a width of the slit is smaller than the diameter of the spacer.
Therefore, this construction has a merit that it is possible to improve the uniformity of the cell gap by effectively functioning an increased number of spacers as the gap maintaining members, since the spacers are prevented from dropping in the slit of the bottom-up pattern layer.
A fabrication method for fabricating the liquid crystal display panel according to the present invention will be described next.
The first fabrication method of the present invention, for fabricating the above described liquid crystal display panel comprises, basically, the patterning steps of forming at least switching elements, wiring lines and a light shielding layer on a transparent substrate, the leveling layer coating step of forming a leveling layer on a surface of a wafer on which the patterns are formed in the patterning steps, the step of forming pixel electrodes on the leveling layer and the step of providing a seal containing spacers in mixed state. In this basic fabrication method of the liquid crystal display panel, the present invention is featured by that a bottom-up pattern layer for increasing height of an underlying layer of the leveling layer is formed in a seal area on the transparent substrate. Further, the present invention is featured by the etching step of forming contact holes in the leveling layer by etching, before the pixel electrodes are formed.
According to the second fabrication method of the present invention, the basic fabrication method is featured by that a portion of the leveling layer, which is coated on the seal area is removed by etching.
According to the third fabrication method of the present invention, the fabrication method according to the first aspect is featured by that a portion of the leveling layer, which is coated on the seal area is removed by etching.
According to the fourth fabrication method of the present invention, the first fabrication method is featured by that the bottom-up pattern layer is formed by utilizing any one of the patterning steps.
According to the fifth fabrication method of the present invention, the fabrication method according to the first fabrication method is featured by that the bottom-up pattern layer is formed by utilizing the light shielding layer forming step of the patterning steps.
According to the sixth present fabrication method of the present invention, the second or third fabrication method is featured by that the leveling layer coated on the seal area is etched away by the etching step of forming the contact holes.
According to the sixth fabrication method of the present invention, there is a merit that the fabrication cost is not increased since the existing etching process is utilized.